This invention relates, in general, to electron optical systems and more particularly to a field emission gun for producing a beam of charged particles, such as ions or electrons.
In U.S. Pat Nos. 3,678,333 and 3,766,427, and Application Ser. No. 225,970 filed Feb. 14, 1972 in the name of Vincent J. Coates and Leonard M. Welter, (all commonly assigned to the assignee of the present application) there are described scanning electron microscopes employing field emission guns in which the present invention may be advantageously embodied. These aforementioned patents and applications are herein incorporated by reference for a fuller understanding of the present invention.
The utilization of the field emission gun incorporating such as a cold field emitting tip under high vacuum permits the formation of a high intensity focused beam of charged particles (e.g. electrons) as an illuminating vehicle for scanning electron microscopy. The charged particle gun of the referenced patents and application provides the high vacuum environment and voltage discharge protection which is required for stable field emission microscopy.
In implementing this high voltage protection, the inclusion of such as shield electrodes placed about the peripheral region of the tip, a separate field operating electrode placed in juxtaposition to the tip, and various structural forms of field electrodes (individually or in combination) were included in the preferred forms of field emission guns.
The operation of these described highly stable, reliable systems is often as a self-focusing electron accelerating system without the use of additional de-magnification lenses so common in thermionic electron microscopy. In such embodiments, the field emission gun operates to form a focused image of the electron beam in a preselected image plane without additional lenses other than those electrodes forming the main focusing and accelerating anodes of the basic field emission gun. In those embodiments of field emission generators utilized in scanning electron microscopy the charged particle beam is focused in a plane occurring on the surface of a specimen to be examined and scanned over the surface area to be investigated by driving the beam by deflection means in a raster pattern. The impacting of the charged particles of the beam upon the specimen causes, in the case of an electron beam, electrons to be scattered, reflected and emitted from the general surface of the specimen under bombardment. Usual scanning electron microscopy techniques includes detecting one or more of the various types of electrons exiting the surface of the specimen by means such as a scintillator detector, the response of which is portrayed on a recording device such as a cathode ray tube, film or the like.
In field emission scanning microscopy, the beam intensity is sufficiently high such that "real time" viewing of the specimen surface may be enjoyed by synchronizing the electron beam directly to the writing beam of a television-type CRT and modulating that beam by the output of the detector.
In the system as described above, the quality of the image viewed of the surface of the specimen is closely related to the detection of the selected types of charged particles exiting the surface of the specimen to the exclusion of other particles -- both of different type or similar but perhaps, not exiting the surface of the specimen in direct response to the bombardment of the surface by the field emission beam. It is herein theorized that wherein a field emission gun is used as an electron source (although the invention concept applies to any charged particle system) the system consists of a field emission tip.sup.10, a first anode.sup.12, and a power supply (V.sub.1) to pull electrons from the tip (see FIG. 1). The electrons which are generated at the tip pass through the aperture in the first anode.sup.12 may be accelerated or decelerated by the next electrode (second anode.sup.14) depending upon the magnitude of V.sub.2. In many cases V.sub.2 &gt;V.sub.1 which means that the electrons passing through the aperture in the first anode are accelerated. Many electrons from the tip strike the first anode, however, and produce secondary and backscattered electrons at this surface. When V.sub.2 is greater than V.sub.1, many of these secondary and backscattered electrons find their way into the accelerating field region caused by the cooperative focusing and accelerating influence of the first and second anodes; for example, an electric field can fringe through the hole in the first anode caused by the effective voltage applied between the first and second anode. The fringing field collects many of the secondary electrons produced on the surface of the first anode and accelerates them toward the second anode (see FIG. 2). The primary electron beam (beam energy .perspectiveto.eV.sub.2) can be focused to a very small probe size as described in the foregoing references. The secondary beam (beam energy .perspectiveto.e(V.sub.2 -V.sub.1) produces a very diffuse "spray" of secondary electrons. In addition, this secondary spray of electrons has an energy different from the primary beam. The number of undesired diffuse secondary electrons, I have found, can be as high as that of the desired primary electrons. The primary beam of electrons is used for many different applications such as previously described, most of which require monoenergetic, well-focused electron probes. The described secondary electron spray constitutes "spatial noise" in a scanning electron probe because areas other than those desired are being bombarded by the spray electrons while the well-focused primary probe is irradiating the area of interest. These spray electrons can cause a large increase in background noise when viewing or recording spatial information about the specimen using the various electron, photon, X-ray, etc. detectors. In addition, the secondary spray creates a broad effective energy spread in the beam at the specimen plane which greatly impairs or confuses the detector and degrades the accuracy or examination of physical phenomena which require a monoenergetic source of excitation and detection such as electron spectroscopy and transmission microscopy.
One of the several objects of the invention is the inclusion of means within the field emission electron gun to generate field intermediate the field emission tip and the principle accelerating field to forestall the entry of secondary electrons from the cloud formed around the tip and field forming electrode into the main accelerating field.
One example of these objects is illustrated by the inclusion of a third electrode biased so as to prevent the secondary electrons produced by the unused portion of the tip current, or other stray currents, to pass into the accelerating section of the field emission gun.